Grimrock

I have been trying some modding with the Grimrock 1 dungeon maker and have been attempting some scripting.
I have run across an issue regarding the 'party:setPosition' command which confuses me. It works just fine when triggered with say a wall button. But if I try to trigger it with a pressure plate the whole program crashes and I end up at the desktop. Can anyone tell me why?

A party:setPosition() call cannot execute from that place without failing an assert. You could call it one frame after the party steps on the plate, however.